The present invention relates to an improved process for urea production, as well as to a method for retrofitting pre-existing urea production plants in which a reaction mixture leaving an urea synthesis reactor is submitted to a stripping step with carbon dioxide (Stamicarbon process).
The invention also relates to a plant for implementing the above mentioned process.
As is well known, the need often arise of increasing the urea production capacity of a pre-existing plant.
To this end, various methods of enhancing the production capacity have been proposed heretofore, such as that described in European Patent application EP-A-0 479 103 by the same applicant.
EP-A-0 479 103 discloses a process wherein highly pure ammonia and carbon dioxide are reacted in a first reaction space, the reaction mixture thus obtained is fed to a recovery section and a synthesis reaction between less pure reagents, substantially recycled by the recovery section (recovery mixture), is carried out.
The need of increasing the urea production capacity as well as of reducing energy consumption is particularly felt in the case of existing plants according to the Stamicarbon Process (STC), including a CO.sub.2 stripping section.
The Stamicarbon Process is described in an article by Zardi in "Nitrogen", No. 135, pp. 28-29 (1982). The Stamicarbon Process is described in an article by Zardi in "Nitrogen", No. 135, pp. 28-29 (1982), in an article by Kaasenbrood et al in "Proceedings No. 166, The Fertilizer Society", London (1977), and in an article in European Chemical News Urea Supplement, Jan. 19, 1969, pp. 17, 19 and 20. It employs an ammonia/carbon dioxide mole ratio in the reactor of 2.8 as the optimum ratio.
These kind of plants, widely used for urea production, afford on the one hand a very simple plant layout, but on the other hand have a very limited operating flexibility that render their retrofitting very difficult in terms of capacity increase and energy consumption reductions.
For instance, it is well known that capacity increases over the 10-20% range are difficult to obtain and anyhow, the elevated cost of the investment would make it economically unfeasible.
The aim of this invention is therefore that of providing an improved process for urea production allowing to encrease the capacity and reduce the energy consumption of CO.sub.2 stripping plants, for instance of the STC type, at low cost and with minimum modifications of the plant layout.